The steady advances in microelectronics are causing structures to shrink continuously, even as more and more components are being place on a given area. This trend is also evident in the development of larger and larger data memories. The classic silicon-based semiconductor technology is nearing its limits for physical and financial reasons, and will soon not be able to keep pace with the drive towards miniaturisation. Components that are being manufactured today have structure sizes of several tens of nanometres. New concepts and materials are needed that will enable the structure sizes and thus entire components to be shrunk yet further, to a few nanometres.
The demand for novel, inexpensive electronics, preferably providing functionalities on flexible substrates continues to grow. For example applications such as intelligent admission cards, extremely inexpensive transponder labels or electronics integrated in clothing are conceivable. Besides their other requirements, all such applications also need memory components. Microelectronics based on crystalline semiconductors can only offer a limited level of functionality for these.
Passive storage concepts have the advantage of a relatively simple construction and the ability to be integrated easily in 3D concepts. Resistive storage concepts, that is to say memories that can assume various electrical resistances and thus store information content, are viewed as promising for purposes of mass storage in the future because of their scalability to the molecular magnitude. A simple construction in crossbar technology enables these components to be produced cheaply and integrated in 3D concepts. One disadvantage of this construction is that it is susceptible to crosstalk with adjacent cells when programming or deleting individual elements. In order to prevent this, and to enable larger memory arrays to be produced, additional active and passive components are necessary. One option consists in connecting each memory cell individually to a zener diode. Thus, crosstalk is prevented by the strongly non-linear characteristic curve. Zener diodes are easy to implement and are used widely in classic silicon technology to stabilise voltages and protect important modules from destruction.
These diodes behave like normal diodes in the forward bias direction, but in the reverse bias direction their resistance suddenly falls dramatically above a certain voltage, the breakdown voltage. The breakdown voltage can be adjusted from 3 to 100V by selectively changing the doping of the electron-conducting layer and/or the hole conducting layer and the modification this brings about in the width of the depletion layer. Zener diodes are currently also used in passive matrix memories. Since these crossbar memories are theoretically scalable down to the molecular level, silicon technology will shortly reach its limits in this field as well.
Accordingly, the search for alternative methods and materials to replace the classic silicon technology is being conducted intensively all over the world.
Organic electronics has emerged as a promising alternative to silicon-based electronics. Among its advantages is the fact that it involves relatively simple processes such as printing or vapour deposition at low temperatures, the ability to work on flexible substrates, and the wide variety of molecular materials.
The filed of organic electronics is having its first applications in organic light emitting diodes (OLEDs).
Following a relatively short development period, these can already be found in many devices. Even now, in the research stage, the efficiencies of these OLEDs are reaching record values that most other light sources cannot rival. The development of OLEDs provides an indication of the potential that is as yet untapped in organic electronics. However, before organic electronics can be treated as a fully developed system, it is necessary to produce not only light emitting diodes but also organic transistors, organic memories and other components in order to take full advantage of the cost benefit in production and to avoid having to rely on a combination of organic electronics and classic silicon technology. Besides organic transistors, organic solar cells are the subject of considerable research efforts all over the world. Although they do not yet offer the same efficiency levels as classic solar cells, they are easy to produce, and as such have the potential for an enormous cost advantage over silicon solar cells. As the number of components increases, components that protect the primary electronics from external influences are needed in organic electronics as well. Voltage stabilisation and overvoltage protection are important considerations, among others.
A number of organic thin film zener diodes consisting of one or more organic layers are known. Several different approaches for such diodes are described in US 2004/0051096 A1. Up to three organic layers of various materials are applied between two electrodes. The zener voltage can be adjusted through appropriate selection of the organic material, electron-conducting (n-conducting) or hole-conducting (p-conducting) for example. The zener voltage can be changed by altering the sequence of layers of organic materials. This document will also show that different zener voltages also result from different electrodes. With the appropriate selection of material, it is possible to achieve zener voltages in the range from 0.1V to 7V. If a specific zener voltage is required, it is possible with a suitable combination of organic material, electrodes, and layer structure. At the same time, however, the current-voltage curve is also altered in the forward direction, which represents a significant drawback. In the forward direction, it is desirable for the diode behaviour to remain as consistent as possible for different zener voltages. Another disadvantage is that only certain electrode materials and combinations can be used for a given zener voltage. This places marked limitations on design freedom.
Another problem with this design is the poor electrical contact properties between the electrodes and the organic material. Injection of charge carriers is hindered by large barriers for electrons as well as holes at the respective boundary surfaces between the organic layers and the metal contacts.
Finally, electrical conductivity in undoped layers is highly sensitive to the layer thickness (a cubic dependency is expected under the precondition of ohmic injection: M. A. Lampert et. al, Current injection in solids, Academic, New York, 1970). As a result, the approaches in production described in US 2004/0051096 A1 are vulnerable to inconsistencies in the production process.
The object of the invention is to provide an improved zener diode, of simple construction and offering improved performance in conjunction with the breakdown voltage. The zener diode should demonstrate stable, reproducible behaviour, and it should be possible to adjust the breakdown voltage without altering the forward bias characteristic curve.